Standing out from the crowd: Are patagial wing tags a potential predator attraction for harriers (Circus spp.)?

Wing tags have been used on many bird species to facilitate individual recognition, although tags are not only conspicuous for humans but may also attract the attention of potential predators. During a Peregrine Falcon Falco peregrinus monitoring program (1997-2011) we collected prey remains from the nests of 37 Peregrine territories in the Basque Country, Northern Spain. We identified 3,127 prey items representing 132 bird species. In the 2009 breeding season, we found, for the first time, four Hen Harrier Circus cyaneus remains and the two wings with orange wing tags of a Montagu's Harrier Circus pygargus in one nest, 1 year later we found in the same nest eight Montagu's Harrier remains, one of them with wing tags, and in 2011 we found eight more Montagu's Harriers, one of them marked. Simultaneously, in the breeding season of 2010, the remains of another Montagu's Harrier were found in other Peregrine nest and two wings with orange and blue wing tags in the perch of a third pair. Our data thus shows an increase of predation rate of harriers by Peregrines; we discuss whether this could be related to the increasing use of wing tags in this species, and thus whether wing tags may have potential negative effects on the birds. We argue that further studies about the impact of identification techniques, and wing-tags in particular, on the survival of target species should be carried out.